1. Field of the Invention
The present invention relates to a dispersion stabilizer. The invention also relates to a dispersion stabilizer for suspension polymerization of a vinyl compound, which exhibits excellent suspension polymerization stability, can produce vinyl polymer granules having a high bulk density through suspension polymerization of a vinyl compound, and can prevent a milky turbidity of waste water after suspension polymerization.
2. Discussion of the Prior Art
The production of a vinyl polymer such as a vinyl chloride resin has been widely conducted industrially by suspension polymerization in which a vinyl compound such as vinyl chloride is dispersed in an aqueous medium in the presence of a dispersion stabilizer and polymerization is conducted using an oil-soluble initiator. In general, the factors governing the quality of the resulting vinyl polymer include conversion, water/monomer ratio, polymerization temperature, type and amount of initiator, type of polymerization vessel, stirring rate and type of dispersion stabilizer. Among these factors, the type of dispersion stabilizer has a significant effect.
Vinyl chloride resin is a general-purpose resin, that is cost effective and offers several advantageous physical and chemical properties. To achieve high throughput and processing productivity it is desirable for the resin to have a high bulk density and excellent processability.
Relating to a method for obtaining a resin having a high bulk density and an excellent processability, Japanese Patent Laid-Open No. 259,609/1996 discloses a method using a dispersion stabilizer, which is made of a modified polyvinyl alcohol having an ethylene unit content of 1 to 24 mol % and a degree of saponification of more than 80 mol %. Japanese Patent Laid-Open Nos. 241,308/1997 and 251,311/1998 disclose a method using a dispersion stabilizer which comprises at least one kind of partially saponified polyvinyl acetate having a degree of saponification of 85 mol % or more. The above-mentioned methods provide resins of high bulk density but are problematic in that waste water formed after suspension polymerization is milky turbid and the chemical oxygen demand (COD) of the waste water is high. Japanese Patent Laid-Open No. 309,602/2000 proposes a method using a dispersion stabilizer made of a vinyl alcohol polymer and an alkali metal salt, however the bulk density and sharpness of the particle size distribution of the resin are not satisfactory.
Accordingly, it is an object of the invention to provide a dispersion stabilizer for suspension polymerization of vinyl compounds, which exhibits excellent suspension polymerization stability, can produce resin having high bulk density, and can prevent the formation of milky turbid waste water after suspension polymerization.
The inventors have found that the foregoing object is attained by a dispersion stabilizer for suspension polymerization of a vinyl compound, which comprises a modified saponified vinyl ester copolymer (A) having an ethylene unit content of 1 to 20 mol %, a degree of saponification of 90 mol % or more and a degree of polymerization of 100 to 3,000, a saponified vinyl ester polymer (B) having a degree of saponification of 60 to 90 mol % and a degree of polymerization of 600 to 4,000, and a saponified vinyl ester polymer (C) having a degree of saponification of 30 to 60 mol % and a degree of polymerization of 100 to 600, and in which the weight ratio component (A)/{component (B)+component (C)} is 5/95 to 40/60.